This invention relates to polycarbonates, and more particularly to a method for enhancing their crystallinity.
Solid state polymerization of polycarbonates is disclosed, for example, in U.S. Pat. Nos. 4,948,871, 5,204,377 and 5,214,073, the disclosures of which are incorporated herein. It involves three steps: a first step of forming a prepolymer, typically by melt polymerization (i.e., transesterification) of a dihydroxyaromatic compound such as bisphenol A with a diaryl carbonate such as diphenyl carbonate; a second step of crystallizing the prepolymer; and a third step of building the molecular weight of the crystallized prepolymer by heating to a temperature between its glass transition temperature and its melting temperature. Use of this polymerization method is of increasing interest by reason of its effectiveness and environmental benefits.
The second or crystallization step of this method is performed, according to said patents, by solvent treatment or heat treatment. As described, the solvent treatment method may in fact employ a good or poor solvent for the prepolymer, with contact involving either the liquid or vapor form thereof. Illustrative "solvents" include aliphatic aromatic hydrocarbons, ethers, esters, ketones and halogenated aliphatic and aromatic hydrocarbons. It is considered important that the "solvent" be allowed to permeate into the prepolymer. The described methods of crystallization apparently require that the crystallinity of the entire prepolymer mass be increased to a value of at least 5% as determined from powder X-ray diffraction patterns.
Copending, commonly owned application Ser. No. 08/986,448 describes a solid state polymerization method which requires only surface crystallization of the precursor polycarbonate. Surface crystallization is effected by contact, typically at a temperature in the range of about 20-50.degree. C., with at least one dialkyl carbonate, optionally in the presence of another non-solvent such as water or a C.sub.1-4 alkanol.
A problem frequently encountered in these SSP processes is interadhesion of the polycarbonate particles prior to the SSP step, which inhibits polymerization. Another problem is that prepolymers in the form of pellets, which are a common physical form in which they are collected, often cannot be efficiently polymerized or disintegrate to fines during the SSP step.
It would be desirable, however, to employ non-solvents which are not as expensive and difficult to obtain as dialkyl carbonates. It would further be desirable to achieve crystallization to the desired degree by a method which can be performed on readily available precursor polycarbonate pellets and the like.